The present invention relates generally to polar modulation transmitters, and more particularly to dynamic compensation for delay mismatch between amplitude and phase signals of polar modulation transmitters.
Due to packet switched data traffic and the need for increased data rates for wide-area wireless connections, a GPRS (General Packet Radio Service) standard was developed to enable transmission of packet data using existing GSM (Global System for Mobile communication) networks. As a result, GPRS mobile terminals may continue to use the relatively constant-envelope transmitter chains typically used for GSM. In addition, to obtain higher bit-rates for EDGE (Enhanced Data rates for GSM Evolution) service, the modulation format was changed from GMSK (Gaussian pre-filtered Minimum Shift Keying) to the more bandwidth efficient (3π/8)-8PSK (8-ary Phase Shift Keying with 3π/8 symbol-to-symbol phase rotation) with a pre-filtering function that enables the spectral properties of the transmit waveform to remain largely unchanged.
Cartesian modulation transmitters and polar modulation transmitters are conventionally used to implement the desired modulation format. Cartesian modulation transmitters generate In-phase (I) and Quadrature-phase (Q) signals that are valid and complete baseband representations of the transmit waveforms. Such signals may be jointly translated directly to a radio frequency (RF) carrier (i.e. the desired transmit channel) by e.g. commonly used linear-IQ up-conversion transmitters.
In the case of a polar modulation transmitter, a Cartesian-to-Polar converter may transform the Cartesian I and Q coordinates into polar coordinates according to Equation (1):Tx(t)=I(t)+i*Q(t)=R(t)*eiθ(t)′  (1)where R(t) represents an amplitude signal comprising the momentary amplitude of the transmit waveform (the envelope of the RF transmit signal), and θ(t) represents a constant-envelope IQ phase signal. Because the amplitude and phase are now separate components, the polar modulation transmitter may advantageously superimpose amplitude and phase modulation onto the RF carrier independently.
Polar modulation transmitters are characterized by the fact that in at least the analog portion of the transmitter circuitry, the to-be-transmitted signals are represented as amplitude and phase signals. Because the amplitude and phase signals travel different paths in the transmitter circuitry, the amplitude and phase signals may experience different path delays. As a result, there may be a delay mismatch between the amplitude and phase signals at the power amplifier, the point where the phase and amplitude signals are recombined.
Delay mismatch may degrade the Adjacent Channel Power Ratio (ACPR). According to current standards, an ACPR less than or equal to −54 dB at 400 kHz and less than or equal to −60 dB at 600 kHz is required. The ideal spectrum (see FIG. 2) meets these requirements with margin. Further evaluation of the ACPR at the EDGE specification frequencies 400 kHz (ACPR≦−54 dB) and 600 kHz (ACPR≦−60 dB) shows that a delay mismatch between the phase and amplitude waveforms of up to 2% of the symbol time may be tolerated. However, when the delay mismatch exceeds 2%, both ACPR specification requirements are violated. Because conventional polar modulation transmitters typically introduce a delay mismatch of approximately 4%, delay mismatch compensation is necessary to meet current standard requirements.